Method for producing acrylic fiber

ABSTRACT

A method for producing acrylic fiber includes preparing a spinning solution including an acrylic copolymer, dimethyl sulfoxide, water, and a polyphosphate; and subjecting the spinning solution to wet spinning, wherein the acrylic copolymer includes acrylonitrile in an amount of 20 to 85 mass %, vinyl chloride in an amount of 14.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 10 mass % with respect to a total mass of the acrylic copolymer.

TECHNICAL FIELD

One or more embodiments of the invention relate to a method for producing acrylic fiber in which a spinning solution is subjected to wet spinning, the spinning solution being obtained by dissolving an acrylic copolymer obtained through copolymerization of acrylonitrile, vinyl chloride, and a sulfonic acid group-containing monomer in dimethyl sulfoxide.

BACKGROUND

Acrylic fiber, especially, acrylic fiber constituted by an acrylic copolymer comprising vinyl chloride or a vinylidene chloride as one of the copolymer components is suitably used as fiber for artificial hair used in a head decoration product such as a wig, hairpiece, or weaving due to its soft texture. In general, acrylic fiber used as artificial hair is produced through wet spinning using a spinning solution obtained by dissolving an acrylic copolymer in a good solvent such as dimethyl sulfoxide. For example, Patent Document 1 discloses that acrylic fiber constituted by an acrylic copolymer constituted by acrylonitrile, vinylidene chloride, and a sulfonic acid group-containing vinyl monomer is produced using a wet spinning method in which a good solvent is used.

However, Patent Document 1 is problematic in that vinylidene chloride is used as one of the copolymer components that is copolymerized with acrylonitrile, and has poor curl setting properties with hot water. In view of this, in order to increase the curl setting properties with hot water, vinyl chloride has been used as one of the copolymer components that are copolymerized with acrylonitrile.

Patent Document

[Patent Document 1] JP 2002-315765A

However, the inventors have found that when a highly-safe dimethyl sulfoxide was used as the good solvent and a spinning solution obtained by dissolving, in dimethyl sulfoxide, an acrylic copolymer obtained through copolymerization of acrylonitrile, vinyl chloride, and a sulfonic acid group-containing monomer was subjected to wet spinning to produce acrylic fiber, the acrylic fiber discolored to yellow or brown.

SUMMARY

One or more embodiments of the present invention provide a method for producing acrylic fiber, according to which, even if a spinning solution obtained by dissolving an acrylic copolymer comprising vinyl chloride as a copolymer component in dimethyl sulfoxide is subjected to wet spinning, it is possible to suppress discoloration of the obtained acrylic fiber.

One or more embodiments of the present invention relate to a method for producing acrylic fiber in which a spinning solution obtained by dissolving an acrylic copolymer in an organic solvent is subjected to wet spinning, in which the acrylic copolymer comprises acrylonitrile in an amount of 20 to 85 mass %, vinyl chloride in an amount of 14.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 10 mass % with respect to the total mass of the acrylic copolymer, the organic solvent is dimethyl sulfaxide, and the spinning solution comprises water and a polyphosphate.

In one or more embodiments, the spinning solution comprises the polyphosphate in an amount of 0.05 to 5 mass % with respect to the total mass of the acrylic copolymer.

The polyphosphate may be at least one compound selected from the group consisting of a pyrophosphate, a tripolyphosphate, a tetrapolyphosphate, a trimetaphosphate, and a tetrametaphosphate, and in one or more embodiments, is a tripolyphosphate.

According to one or more embodiments of the present invention, it is possible to provide acrylic fiber in which discoloration into yellow or brown is suppressed even if a spinning solution obtained by dissolving an acrylic copolymer comprising vinyl chloride as a copolymer component in dimethyl sulfoxide is subjected to wet spinning.

DESCRIPTION OF THE EMBODIMENTS

In order to prevent the acrylic fiber discoloring to yellow or brown when a spinning solution obtained by dissolving, in dimethyl sulfoxide (DMSO), an acrylic copolymer comprising acrylonitrile, vinyl chloride, and a sulfonic acid group-containing monomer is subjected to wet spinning, the inventors conducted intensive studies and found that by adding water and a polyphosphate to the spinning solution, discoloration of the acrylic fiber to yellow or brown was suppressed while having good spinning properties. It is inferred that when the spinning solution obtained by dissolving the acrylic copolymer into dimethyl sulfoxide is subjected to wet spinning, in the producing step, the acrylic copolymer undergoes dehydrochlorination, dimethyl sulfoxide is decomposed by the produced hydrochloric acid, and thereby the fiber discolors to yellow or brown. If a step of dissolving the acrylic copolymer into dimethyl sullfxide is performed at a temperature higher than room temperature for a long period of time, the acrylic copolymer significantly undergoes dehydrochlorination. In general, the polyphosphate is used as a phosphorus flame retardant for increasing the flame-retardancy of fiber, and surprisingly, the inventors found that by adding a polyphosphate together with water to a spinning solution obtained by dissolving, in dimethyl sulfoxide, an acrylic copolymer obtained through copolymerization of acrylonitrile, vinyl chloride, and a sulfonic acid group-containing monomer and subjecting the resulting mixture to wet spinning, discoloration of the acrylic fiber to yellow or brown was suppressed. Although the mechanism by which the discoloration of the acrylic fiber is suppressed by adding the polyphosphate together with water to a spinning solution and performing wet spinning is not clear, it is inferred that the suppression was caused by hydrochloric acid (hydrogen ions) produced through dehydrochlorination of the acrylic copolymer being utilized in an equilibrium reaction with a phosphate group derived from the polyphosphate that dissolved in water, whereby DMSO is prevented from being decomposed by hydrochloric acid.

The acrylic copolymer comprises acrylonitrile in an amount of 20 to 85 mass %, vinyl chloride in an amount of 14.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 10 mass % with respect to the total mass of the acrylic copolymer. When the acrylic copolymer comprises vinyl chloride in an amount of 14.5 to 79.5 mass %, the fiber has good flame-retardancy. When the acrylic copolymer comprises acrylonitrile in an amount of 20 to 85 mass %, the fiber has good heat resistance and a processing temperature during curl setting can be set as appropriate. Due to the acrylic copolymer comprising the sulfonic acid group-containing monomer in an amount of 0.5 to 10 mass %, its hydrophilicity increases and void fraction decreases. The acrylic copolymer may comprise acrylonitrile in an amount of 20 to 80 mass %, vinyl chloride in an amount of 19.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 5 mass %, and in one or more embodiments, comprises acrylonitrile in an amount of 20 to 75 mass %, vinyl chloride in an amount of 24.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 5 mass %.

Although there is no particular limitation on the sulfonic acid group-containing monomer, for example, allylsulfonic acid, methallylsulfonic acid, styrenesulfonic acid, isoprenesulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid, and metal salts such as sodium salts thereof: amine salts thereof and the like can be used thereas. The sulfonic acid group-containing monomer can be used alone or in combination of two or more.

The acrylic copolymer is dissolved in dimethyl sulfaxide. By using dimethyl sulfrxide as an organic solvent, its safety increases.

From the viewpoint of effectively suppressing discoloration of the acrylic fiber, the spinning solution may comprise a polyphosphate in an amount of 0.05 mass % or more, or in an amount of 0.06 mass % or more, or in an amount of 0.07 mass % or more with respect to the total mass of the acrylic copolymer. From the viewpoint of spinning properties and prevention of discoloration, with regard to the upper limit of the polyphosphate, the spinning solution may comprise the polyphosphate in an amount of 5 mass % or less, or in an amount of 4.5 mass % or less, or in an amount of 4 mass % or less, or in an amount of 3.5 mass % or less, or in an amount of 3 mass % or less, or in an amount of 2.5 mass % or less, or in an amount of 2 mass % or less, or in an amount of 1.5 mass % or less, or in an amount of 1 mass % or less, or in an amount of 0.9 mass % or less, or in an amount of 0.7 mass % or less, or in an amount of 0.5 mass % or less with respect to the total mass of the acrylic copolymer.

The polyphosphate need only be an inorganic polyphosphate, and has no limitation. For example, pyrophosphates, tripolyphosphates, tetrapolyphosphates, trimetaphosphates, tetrametaphosphates, and the like can be used. Among these, from the viewpoint of a high solubility and ease of acquisition, tripolyphosphates may be preferable. There is no particular limitation on the type of salt, and any salts such as sodium salts, potassium salts, and ammonium salts can be used. From the viewpoint of excellent solubility in water, salt may be a watersoluble salt such as a sodium salt and a potassium salt. Examples of the pyrophosphates (also referred to as “diphosphate”) include sodium pyrophosphate and potassium pyrophosphate. Examples of the tripolyphosphates include sodium tripolyphosphate, aluminum dihydrogen tripolyphosphate, and potassium tripolyphosphate. Examples of the tetrapolyphosphates include sodium tetrapolyphosphate and potassium tetrapolyphosphate.

From the viewpoint of spinnability and solubility of the polyphosphate, the spinning solution may comprise water in an amount of 8 to 16 mass %, or in an amount of 8 to 15 mass %, or in an amount of 8 to 14 mass %, or in an amount of 8 to 13 mass %, or in an amount of 8 to 12.5 mass % with respect to the total mass of the acrylic copolymer.

Although the composition of the spinning solution depends on the composition of the acrylic copolymer, the spinning solution may comprise, with respect to the total mass of the spinning solution, the acrylic copolymer in an amount of 20 to 30 mass %, DMSO in an amount of 65.2 to 78.49 mass %, water in an amount of 1.5 to 4.8 mass %, and the polyphosphate in an amount of 0.01 to 1.5 mass %, or comprises the acrylic copolymer in an amount of 22 to 30 mass %, DMSO in an amount of 66 to 75.99 mass %, water in an amount of 2 to 4 mass %, and the polyphosphate in an amount of 0.01 to 1.5 mass %, or comprises the acrylic copolymer in an amount of 25 to 30 mass %, DMSO in an amount of 66.5 to 72.49 mass %, water in an amount of 2.5 to 3.5 mass %, and the polyphosphate in an amount of 0.01 to 1.5 mass %, for example.

The spinning solution may also comprise other additives for improving fiber properties as needed. Examples of the additives include gloss adjusting agents such as titanium dioxide, silicon dioxide, esters and ethers of cellulose derivatives including cellulose acetate, coloring agents such as organic pigments, inorganic pigments, and dyes, and stabilizers for increasing light resistance and heat resistance.

The spinning solution, although not particularly limited, can be prepared by mixing the acrylic copolymer, DMSO, water, and the polyphosphate. From the viewpoint of the solubility of the polyphosphate, the spinning solution may be prepared by mixing the acrylic copolymer, DMSO, and an aqueous solution of the polyphosphate. In order to increase the solubility of the acrylic copolymer, the spinning solution may be prepared by, after DMSO and the aqueous solution of the polyphosphate are added to the acrylic copolymer, stirring the resulting mixture at 40 to 80° C. for 3 to 12 hours, or may be prepared by stirring the resulting mixture at 50 to 75° C. for 4 to 10 hours, or may be prepared by stirring the resulting mixture at 60 to 70° C. for 5 to 8 hours. Note that it is inferred that in the preparation of the spinning solution, if a mixture of the acrylic copolymer, DMSO, and the aqueous solution of the polyphosphate is processed at 60° C. or more for 5 hours or more, for example, hydrochloric acid may be produced through dehydrochlorination of the acrylic copolymer, but the produced hydrochloric acid (hydrogen ions) is utilized in an equilibrium reaction with a phosphate group derived from the polyphosphate, and thereby decomposition of DMSO by hydrochloric acid is prevented and discoloration of fiber to yellow or brown is suppressed. An acrylic copolymer comprising moisture may also be used as the acrylic copolymer. When the acrylic copolymer comprising moisture (hereinafter, also simply referred to as watercomprising acrylic copolymer) is used as the acrylic copolymer, in one or more embodiments of the present invention, the total mass of the acrylic copolymer refers to the dry mass of the acrylic copolymer, and the dry mass of the acrylic copolymer is obtained by drying a watercomprising acrylic copolymer at 60° C. for 10 hours, removing moisture, and then measuring the resulting mass thereof. DMSO comprising moisture may be used as DMSO. In this specification, unless otherwise stated, the acrylic copolymer refers to an acrylic copolymer that comprises no moisture, and DMSO refers to DMSO that comprises no moisture.

Acrylic fiber is obtained by wet spinning the spinning solution with an ordinary method. For example, first, the spinning solution is discharged directly or through a spinning nozzle into a coagulation liquid (coagulation bath) with an aqueous solution of DMSO, and coagulated so as to obtain fiber. The coagulation bath can be prepared using an aqueous solution of DMSO having a DMSO concentration of 40 to 70 mass % at a temperature of 5 to 40° C., for example. If the concentration of the good solvent such as DMSO in the coagulation bath is excessively low, the spinning solution tends to coagulate fast, its coagulated structure tends to be rough, and voids tend to form in the fiber.

Next, the fiber (coagulated yarn) may be introduced into an aqueous solution of DMSO whose concentration is lower than that of the coagulation liquid and that has a temperature of 30° C. or more, or into warm water having a temperature of 30° C. or more, may be subjected to solvent removal, water washing, and drawing, and relaxing may also be performed after drawing as needed. In one or more embodiments, after the fiber is drawn in the aqueous solution of DMSO whose concentration is lower than that of the coagulation liquid and that has a temperature of 30° C. or more, the fiber is washed with warm water having a temperature of 30° C. or more. The solvent is removed by water washing. From the viewpoint of increasing the strength and productivity of fiber, the draw ratio, although not particularly limited, may be 2 to 8-fold, or 2 to 7-fold, or 2 to 6-fold. It is inferred that during the steps of solvent removal, water washing, and drawing in the aqueous solution of DMSO as well, if fiber is processed at 80° C. or more for 1 hour or more, for example, hydrochloric acid may be produced through dehydrochlorination of the acrylic copolymer, but the produced hydrochloric acid (hydrogen ions) is utilized in the equilibrium reaction with a phosphate group derived from the polyphosphate, as a result of which decomposition of DMSO by hydrochloric acid is prevented and discoloration of fiber to yellow or brown is suppressed.

Next, the fiber (drawn yarn) may be dried. During drying, a finishing oil may also be attached to the fiber as needed. In general, it is suffcient to use the finishing oil that is used to prevent static electricity, and prevent fiber from sticking each other, and improve texture of fiber in the spinning step, and a known finishing oil can be used. The drying temperature, although not particularly limited, may be 110 to 190° C., and in one or more embodiments, 110 to 160° C., for example. The dried fiber may also be further drawn thereafter as needed, and the draw ratio may be 1 to 4-fold. In one or more embodiments, the total draw ratio including drawing before drying is 2 to 12-fold.

In one or more embodiments, the fiber that is obtained by drying or obtained by drawing after drying may be subjected to relaxing processing at 15% or more. The relaxing processing can be performed in dry heat or a superheated steam atmosphere at a high temperature of 150 to 200° C., and in one or more embodiments, 150 to 190° C., for example. Alternatively, the relaxing processing can be performed in a pressurized water vapor atmosphere or a heated pressurized water vapor atmosphere at 120 to 180° C. and 0.05 to 0.4 MPa, and in one or more embodiments, 0.1 to 0.4 MPa.

From the viewpoint of suitable use as artificial hair, the single fiber fineness of the acrylic fiber may be 30 to 100 dtex, or 40 to 80 dtex, or 45 to 70 dtex. Herein, the single fiber fineness means the average value of the finenesses of any 100 single fibers.

The acrylic fiber may also comprise phosphorus derived from the polyphosphate. For example, the acrylic fiber may also comprise phosphorus in an amount of 5 to 250 ppm, in an amount of 10 to 150 ppm, and in an amount of 15 to 80 ppm with respect to the total mass of the acrylic fiber. The amount of phosphorus can be determined as follows.

EXAMPLES

One or more embodiments of the present invention will be described in further detail using the following Examples. Note that the present invention is not limited to the following Examples.

Example 1

Spinning Solution

500 g of an acrylic copolymer constituted by 45.7 mass % acrylonitrile, 52.3 mass % vinyl chloride, and 2.0 mass % sodium styrene sulfonate, 1223 g of DMSO, and 62.5 g of an aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass % were added to a 7-L stainless steel container, the acrylic copolymer was dissolved through stirring at 70° C. for 12 hours, and thereby a spinning solution was produced.

Spinning Conditions

The obtained spinning solution was subjected to wet spinning at a spinning speed of 2 m/min in a coagulation bath with an aqueous solution of DMSO having a temperature of 20° C. and a concentration of 57 mass %, using a spinning nozzle (hole diameter: 0.3 mm, number of holes: 50 holes), and then drawn 3-fold in a drawing bath with an aqueous solution of DMSO solution having a temperature of 80° C. and a concentration of 50 mass %. Next, after water washing was performed using warm water having a temperature of 90° C., acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained by drying and drawing 2-fold at 140° C., and performing 20%-relaxing processing at 160° C.

Example 2

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that an aqueous solution of sodium tripolyphosphate having a concentration of 1.22 mass % was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Example 3

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that an aqueous solution of sodium tripolyphosphate having a concentration of 2.44 mass % was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Example 4

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that an aqueous solution of sodium tripolyphosphate having a concentration of 3.66 mass % was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Example 5

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that an aqueous solution of sodium pyrophosphate having a concentration of 1.22 mass % was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Example 6

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that an aqueous solution of sodium tetrapolyphosphate having a concentration of 1.22 mass % was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Comparative Example 1

Acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained similarly to Example 1, except that pure water was used instead of the aqueous solution of sodium tripolyphosphate having a concentration of 0.61 mass %.

Comparative Example 2

Spinning Solution

500 g of an acrylic copolymer constituted by 45.7 mass % acrylonitrile, 52.3 mass % vinyl chloride, and 2.0 mass % sodium styrene sulfonate, 1223 g of DMSO, and 0.7625 g of sodium tripolyphosphate were added to a 7-L stainless steel container, the acrylic copolymer was dissolved by stirring at 70° C. for 12 hours, and thereby a spinning solution was produced.

Spinning Conditions

The obtained spinning solution was subjected to wet spinning at a spinning speed of 2 m/min in a coagulation bath with an aqueous solution of DMSO having a temperature of 20° C. and a concentration of 57 mass %, using a spinning nozzle (hole diameter: 0.3 mm, number of holes: 50 holes), and then drawn 3-fold in a drawing bath with an aqueous solution of DMSO having a temperature of 80° C. and a concentration of 50 mass %. Next, after water washing was performed using warm water having a temperature of 90° C., acrylic fiber having a single fiber fineness of approximately 46 dtex was obtained by drying and drawing 2-fold at 140° C., and performing 20%-relaxing processing at 160° C. 50 yarns broke in the coagulation bath in the 2 hours after spinning started, and operability was extremely unstable.

The color of the acrylic fibers of Examples 1 to 6 and Comparative Examples 1 and 2 was measured using the following colorimetry method, and the results are shown in Table 1 below. Also, the content ofphosphorus in the acrylic fibers of Examples 1 to 6 and Comparative Examples 1 and 2 was measured using the following phosphorus quantification method, and the results are shown in Table 1 below. Table 1 below also shows the blending amount of a polyphosphate and the blending amount of water with respect to the total mass of the acrylic copolymer in the spinning solution.

Method for Measuring Color of Fiber

A sample for colorimetry was produced such that the fiber bundle of 500 fibers had a width of 5 mm, colorimetry was performed at any 4 positions using a spectrophotometer (“CM-2600d” produced by KONICA MINOLTA, INC), in conditions of diffused lighting with 10 degrees, light receiving method of D65, measurement diameter of a 8 mm, and an SCE method, and the average value of those measurement values was obtained as the color.

Phosphorus Quantification Method

Sulfuric acid, nitric acid, hydrofluoric acid, and perchloric acid were added to fibers that were cut to a cut length of 0.5 cm, and the fibers were decomposed by heating at 120° C. in reflux conditions. After the resulting mixture was condensed by heating until sulfuric acid produced white smoke, the condensate was dissolved by warming with diluted nitric acid at 100° C. for 45 minutes, and calibrated to a certain volume. The obtained liquid with the certain volume was used to quantify the phosphorus element with an ICP emission spectrophotometer (“SPS4000” produced by Seiko Instruments Inc).

TABLE 1 Blending Content of amount of Blending phosphorus poly- amount of Type of in acrylic phosphate water poly- fiber Color of acrylic fiber (mass %) (mass %) phosphate (ppm) a value b value Ex. 1 0.0763 12.4238 sodium 18 1.30 21.16 Ex. 2 0.1525 12.3475 tripoly- 46 1.82 21.60 Ex. 3 0.3050 12.1950 phosphate 57 2.01 22.00 Ex. 4 0.4575 12.0425 74 1.66 21.92 Ex. 5 0.1525 12.3475 sodium 21 1.44 21.70 pyro- phosphate Ex. 6 0.1525 12.3475 sodium 77 1.72 21.34 tetrapoly- phosphate Comp. — 12.5 — 0 2.63 22.71 Ex. 1 Comp. 0.1525 — sodium not not not Ex. 2 tripoly- measured measured measured phosphate

As is clear from the result of Table 1 above, in wet spinning using the spinning solution obtained by dissolving, in dimethyl sulfoxide, the acrylic copolymer comprising acrylonitrile, vinyl chloride, and a sulfonic acid group-containing monomer, in Examples 1 to 6 in which the spinning solutions contained water and polyphosphates, a values and b values were lower compared to Comparative Example 1 in which no polyphosphate was added to the spinning solution, and in particular, the b value was at least 0.5 lower than that of Comparative Example 1, and discoloration of the acrylic fiber was reduced. Note that although the polyphosphate was added to the spinning solution, in Comparative Example 2 in which no water was added, as described above, yarn broke and spinnability were poor.

Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims. 

What is claimed is:
 1. A method for producing acrylic fiber, comprising: preparing a spinning solution comprising an acrylic copolymer, dimethyl sulfoxide, water, and a polyphosphate; and subjecting the spinning solution to wet spinning, wherein the acrylic copolymer comprises acrylonitrile in an amount of 20 to 85 mass %, vinyl chloride in an amount of 14.5 to 79.5 mass %, and a sulfonic acid group-containing monomer in an amount of 0.5 to 10 mass % with respect to a total mass of the acrylic copolymer.
 2. The method for producing acrylic fiber according to claim 1, wherein the spinning solution comprises the polyphosphate in an amount of 0.05 to 5 mass % with respect to the total mass of the acrylic copolymer.
 3. The method for producing acrylic fiber according to claim 1, wherein the polyphosphate is at least one compound selected from the group consisting of a pyrophosphate, a tripolyphosphate, a tetrapolyphosphate, a trimetaphosphate, and a tetrametaphosphate.
 4. The method for producing acrylic fiber according to claim 2, wherein the polyphosphate is at least one compound selected from the group consisting of a pyrophosphate, a tripolyphosphate, a tetrapolyphosphate, a trimetaphosphate, and a tetrametaphosphate.
 5. The method for producing acrylic fiber according to claim 3, wherein the polyphosphate is a tripolyphosphate.
 6. The method for producing acrylic fiber according to claim 4, wherein the polyphosphate is a tripolyphosphate. 